We are interested in genes expressed during development that regulate the choice of cell fate (eg. nerve, muscle, skin) during development. Our model system is the nematode C. elegans (a small free-living worm) that is widely used for developmental studies because of its small size, ease of culture in the laboratory, simple anatomy, rapid proliferation, and genetic manipulability. One class of genes we study encode transcription factors, proteins that bind DNA directly to regulate the expression of other genes. Two transcription factors we have focused on are CeMyoD and CeE/DA, homologs of vertebrate factors known to act together (as heterodimers) to regulate the development of striated muscle. In collaboration with Dr. Andy Fire (Carnegie Institution of Washington) and Dr. Bruce Paterson (NCI/NIH) we have shown that CeMyoD and CeE/DA function differently in C. elegans embryogenesis than their counterparts do in vertebrate development. For example, CeMyoD and CeE/DA do not heterodimerize efficiently in vitro and CeE/DA is not detectable in differentiating striated muscle cells leading us to suggest that in C. elegans CeMyoD is functioning as a homodimer to regulate downstream genes. In addition, we have recently identified new mutations in CeMyoD that affect tissues in which CeMyoD is not present. The simple anatomy and defined lineage of all cells in C. elegans has allowed us to determine how these CeMyoD mutations affect non-muscle cell fate choices, revealing unexpected and novel functions for CeMyoD. Although CeE/DA is not required for striated muscle cell fate, CeE/DA is likely an important factor regulating other cell fate choices, such as non-striated myogenesis, neurogenesis, and gonadogenesis. In these developmental pathways, our evidence suggests that CeE/DA acts as a transcriptional regulator in much the same way as its vertebrate homologs. We have identified likely heterodimer partners for CeE/DA in each of these tissues and we are currently studying the function and DNA binding site preferences for these interacting factors. One approach to studying CeE/DA function employs the expression of a dominant negative, or toxic, form of CeE/DA that we re-introduce into the animal using transgenic techniques. Our knowledge from other systems allows us to predict the phenotype of animals expressing a toxic form of CeE/DA and our hope is that the simple anatomy of C. elegans will allow us to add new details to how these transcription factors are functioning.